About

ssc2 installs Stata packages from date-based snapshots of the SSC archive, mirrored at ssc-ng/archive. It is a drop-in replacement for the built-in ssc command.

What it does

The SSC archive contains only the most recent version of each user-written Stata package. When an author uploads an update, the previous version is no longer available. This creates challenges for reproducibility: researchers seeking to replicate older analyses may be unable to obtain the package versions originally used.

ssc2 addresses this limitation by adding a date() option to the standard ssc workflow. It serves packages as they existed on a given date using daily snapshots stored in the ssc-ng/archive repository, following the same general approach that dated CRAN snapshots introduced for the R ecosystem (MRAN/checkpoint, Posit Package Manager).

When no date is requested, ssc2 delegates to the official ssc command, ensuring that existing workflows continue to function unchanged. The subcommands new, hot, and uninstall are always delegated.

Snapshots are date-stamped git tags in the mirror repository. Daily snapshots exist from 2021-12-21 onward (with occasional gaps; see the mirror’s ERRATA); three earlier snapshots exist (2017-08-10, 2021-04-15, 2021-08-10).

Installation

Released versions live on the dist branch and are tagged. The latest tag always points at the newest stable release, never at a pre-release.

* The current stable release
net install ssc2, all replace from("https://raw.githubusercontent.com/ssc-ng/ssc2/latest/")

To pin an exact version in a replication package, use the release tag instead of latest:

* A specific release
net install ssc2, all replace from("https://raw.githubusercontent.com/ssc-ng/ssc2/v3.0.0/")

Quick example

// Install the version of reghdfe that was current on 7 January 2022
ssc2 install reghdfe, date(2022-01-07)
which reghdfe    // -> version 5.7.3 13nov2019

// Without date(), identical to -ssc install-
ssc2 install reghdfe, replace

// Most recent mirrored state
ssc2 install reghdfe, date(latest) replace

// Everything else passes through to ssc
ssc2 hot, n(5)

The snapshot picker on the home page writes these commands for you.

Overriding the mirror location

The mirror may move to a different GitHub organization. The base URL is resolved in this order of precedence:

the from() option;
the Stata global SSC2_MIRROR (e.g. global SSC2_MIRROR "https://...", or populated from the environment via global SSC2_MIRROR : environment SSC2_MIRROR);
the environment variable SSC2_MIRROR (read automatically);
the built-in default.

SSC2_MIRROR_API (global or environment variable) analogously overrides the GitHub API endpoint used only to produce better error messages when a snapshot lookup fails.

See the Reference page for full details.

Testing

Run test.do in Stata. It exercises dated installs, delegation to ssc, date(latest), copy/type with dates, and error handling.

Authors

Lars Vilhuber (Labor Dynamics Institute, Cornell University), Ian Joyce (AEA Data Replicator Intern, University of Notre Dame), and contributors.

Citation

A software article is in preparation; until it appears, please cite the GitHub repository.